1. Field of the Invention
The invention relates to a semiconductor device, and more particularly, to a semiconductor device containing ferroelectric (FE) material.
2. Description of the Prior Art
A semiconductor device means any device which can function by utilizing semiconductor characteristics, such as an electro-optical device, a semiconductor circuit, and an electronic device. Accordingly, semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as example.
Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layer, conductive layers, and semiconductor layers over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon. Since the semiconductor integrated circuit industry has experienced rapid growth and improvement, technological advances in semiconductor materials and design have produced increasingly smaller and more complex circuits. Consequently, the number of interconnected devices per unit of area has increased as the size of the smallest components that can be reliably created has decreased. However, as the size of the smallest components has decreased, numerous challenges have risen. As features become closer, current leakage can become more noticeable, signals can crossover more easily, and power usage has become a significant concern.
Typically, when a gate bias of a metal-oxide-semiconductor field effect transistor (hereinafter abbreviated as MOS FET) device is below the threshold voltage Vth, the current flow between the source and the drain, which is defined as the subthreshold current, is supposed to be zero. Or, the subthreshold current was supposed to be very small and thus in early analytical models of the electrical behavior of MOS FET were even assuming a zero off-state current/subthreshold current. Those skilled in the art should have known there is a linear relationship between the subthreshold current and the gate voltage, which is recognized as subthreshold swing (SS). A small subthreshold swing is highly desired since it improves the ratio between the on and off currents, and therefore reduces leakage currents. Using a device with a small subthreshold swing therefore has advantages such as suppression of power consumption due to reduction in operation voltage and reduction in off leakage current. However, the subthreshold swing cannot be less than 60 mV/sec due to the physical limit of MOS FET device in state-of-the-art. Thus, it is still in need to reduce the subthreshold swing despite the physical limit.